If belt-tightening works, Europe will have to replace waning domestic consumption with foreign demand, and a weaker euro will make exports more competitive. Yet "if fiscal-austerity plans don't work, the euro loses credibility. Either way, the euro weakens," says Marc Chandler of Brown Brothers Harriman.

The interest-rate outlook also favors the greenback. The U.S. may have a knack for keeping borrowing costs artificially low and swelling asset bubbles, but even its benevolent central bank will have to consider raising rates as the economy expands for a fourth straight quarter and as employers hire again. In contrast, a big swath of Europe is grappling with sluggish growth, and Spain is plagued with 20% unemployment (see table below). The ECB can't easily boost rates before the U.S. does.

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